Powder metal magnetic pole piece

ABSTRACT

A powder metallurgy produced wear resistant magnetic pole piece for use as a pickup head with magnetic playback tapes is provided made of a magnetically soft ferrous alloy containing effective amounts of silicon and aluminum. A powder of the alloy is oxidized to provide a thin oxide on the particles thereof. The powder is then hot pressed in vacuum at an elevated temperature into a dense sintered body. The presence of oxide in the grain boundaries confers wear resistant properties to the alloy. An oxygen content of about 3,400 to 4,400 ppm is preferred.

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States atent [1 Gabriel et a1.

POWDER METAL MAGNETIC POLE PIECE Inventors: James M. Gabriel, Monsey, N.Y.;

William Reilly, Fort Lee, NJ.

Assignee: Chromalloy American Corporation,

New York, N.Y.

Filed: Dec. 29, 1970 Appl. No.: 102,364

US. Cl. 29/1825, 75/206, 75/212,

Int. Cl. C22c 3/00 Field of Search 29/1825; 75/206,

References Cited UNITED STATES PATENTS 5/1972 Moss 75/206 X June 19, 1973 Primary Examiner-Carl D. Quarforth Assistant ExaminerR. E. Schafer Attorney-Sandoe, Hopgood & Calimafde [57] ABSTRACT A powder metallurgy produced wear resistant magnetic pole piece for use as a pickup head with magnetic playback tapes is provided made of a magnetically soft ferrous alloy containing effective amounts of silicon and aluminum. A powder of the alloy is oxidized to provide a thin oxide on the particles thereof. The powder is then hot pressed in vacuum at an elevated temperature into a dense sintered body. The presence of oxide in the grain boundaries confers wear resistant properties to the alloy. An oxygen content of about 3,400 to 4,400 ppm is preferred.

5 Claims, 4 Drawing Figures POWDER METAL MAGNETIC POLE PIECE This invention relates to a sintered wear resistant powder metal magnetic pole piece and to a method of making the same.

STATE OF THE ART Magnetic playback television tapes employ magnetic pole pieces as pickup heads for playing back pictures recorded thereon. In one design, four pole pieces are employed in an assembly, each pole piece being soldered every 90 within a panel guide wheel near the periphery thereof. The guide wheel rotates about a central axis and wears against a magnetic playback tape of gamma iron oxide. The pickup face of each pole piece, which is disposed at the periphery of the wheel, is the only part that contacts the tape in service. As the pole pieces wear, the gap between the tape and wheel becomes too great for proper magnetic performance (permeability, coercivity, etc.) such that the picture is generally degraded, thus requiring frequent replacement with new pole pieces at, for example, IOO-hour intervals.

The pole pieces are made of a soft magnetic material comprising an iron-base alloy containing approximately 9.4 percent by weight of silicon and 5.7 percent by weight of aluminum. conventionally, the alloy is cast into a desired shape and the individual pole pieces machined from the casting, since the alloy is too brittle to withstand conventional working operations, such as rolling, forging, forming and the like. The casting usually has a coarse grained microstructure of less than ASTM 4 accompanied by limited wear characteristics. It would be desirable to provide a soft magnetic alloy of the foregoing type characterized by improved wear resistance and capable of being employed for prolonged periods of time in frictional contact with magnetic playback tape containing gamma iron oxide.

OBJECTS OF THE INVENTION It is thus the object of the invention to provide a powder metallurgy method for producing magnetic pole pieces characterized by improved wear resistance when in rubbing contact with a magnetic playback tape of gamma oxide iron.

Another object is to provide as an article of manufacture a sintered soft magnetic alloy characterized by improved wear resistance as a magnetic pole piece when employed in rubbing contact with a moving magnetic tape of gamma iron oxide.

These and other objects will more clearly appear when taken in conjunction with the following disclosure and the accompanying drawing, wherein:

FIGS. 1 and 2 illustrate magnetic pole pieces pro duced in accordance with the invention;

FIG. 3 depicts, schematically, one embodiment of a die assembly which is employed in producing hot pressed products in accordance with the invention; and

FIG. 4 is a reproduction of a photomicrograph taken at 500 times magnification showing a typical microstructure oxide surrounding the grains of the magnetic alloy.

STATEMENT OF THE INVENTION One embodiment of the invention resides in a powder metallurgy method of producing a magnetically soft ferrous alloy containing an effective amount of silicon and aluminum ranging from about 2 to 12 percent silicon and from about 2 to 10 percent aluminum for use in the manufacture of magnetic pole pieces characterized by improved resistance to wear when in rubbing contact with a moving magnetic playback tape of gamma iron oxide. A classified powder of the alloy is first thermally oxidized in air at an elevated temperature to provide a thin oxide film at the surface of the particles and the oxidized powder is then hot consolidated to a substantially fully dense body having a fine grained metallographic structure with the oxide film encapsulating the grain boundaries thereof. The oxide film improves wear resistance without substantially adversely affecting the magnetic properties.

The invention also provides as an article of manufacture a consolidated sintered magnetic alloy containing effective amounts of silicon, aluminum, oxygen, and the balance essentially iron suitable for use in magnetic pole pieces for use with magnetic playback tapes.

The magnetic alloy is advantageously produced as a vacuum hot pressed billet from thermally oxidized alloy powder of particle size ranging from about 10 to 20 microns. This results in a fine grained fully dense pressing in which the wear performance is substantially superior to the cast alloy product. The benefit provided by the product is attributed (l) to grain refinement which offers more grain boundary area than the cast structure; and (2) to the presence of a localized oxide film in the grain boundary.

As stated hereinbefore, the powder metallurgy alloy is particularly suitable for magnetic pole pieces of the type shown in FIGS. 1 and 2, FIG. 1 showing a typical panel guide wheel 10 having four magnetic pole pieces (11, l2, l3 and 14) located at intervals around the periphery of the wheel. The actual wear surface of each of the pole pieces, i.e., surfaces 11A, 12A, 13A and 14A, extend slightly beyond the periphery of the wheel as shown. An enlarged view of a pole piece 15 is depicte d with the wear surface 15A indicated as shown.

DETAIL ASPECTS OF THE INVENTION In carrying out the more preferred aspects of the invention, the alloy composition consisting essentially of approximately 9.4 percent silicon (e.g. 9.4% i 0.4%), approximately 5.7 percent aluminum (e.g. 5.7% i 0.3%), and the balance essentially iron, is preferably in the vacuum cast form. The vacuum cast ingots are ground clean, remelted and then atomized using an inert gas (preferably argon) preferably jet controlled using proper processing parameters (i.e., pressure, purity, flow, etc.) to achieve maximum yield of spherical powder in preferably the 10 to 20 micron range.

The material is classified to extract the 10 to 20 micron spherical powder, and the plus 20 micron oversize powder is processed by conventional ballmilling in a stainless steel mill containing a charge of la-inch stainless steel balls. A typical milling procedure consists of a 750 gram charge of the alloy powder dispersed in 1,500 cc of denatured alcohol to act as a refrigerant and to reduce contamination.

Following milling, the milled powder is vacuum dried in a metal desiccator at 25 to 30 inches Hg with the oven temperature not exceeding F. The milled product is then classified to remove the 10 to 20 micron powder. Milling times and rotation speeds are controlled to assure that flake" powder shapes are not developed, otherwise, such powder contributes to an anistropic metallurgical structure and a non-uniform performance in the field.

The milled powder has an irregular shape and is preferably blended with the unmilled atomized spherical powder. Tests have indicated an optimum blend ratio to be 5 parts of milled powder to 1 part of atomized powder. The blend ratio may vary from about 4 to l to as high as about 7 to l. A typical charge of powder is homogeneously mixed for 1 hour prior to pressing. Where intermediate handling and storage times exceed 48 hours, the powder is stored in evacuated containers under a vacuum level of 10" Torr or better.

A 600-gram charge of atomized (spherical shape) and milled (irregular shape) powder is blended and the blended powder is first pre-oxidized at 600 F for 1 hour in air. One method is to uniformly spread the powder of 10 to micron size between two fine mesh stainless steel screens confined within the side walls of a stainless steel tray with side ports to allow the powder to be pre-oxidized similar to a fluid bed system in a recirculating air environment. To assure the desired oxidation, the powder is visually inspected at ten times magnification under stereo binoculars for oxide color uniformity and to assure absence of agglomeration. An oxygen content of about 3,400 to 4,400 ppm by weight has been found very advantageous, although the oxygen content may vary from about 2,000 to 8,000 ppm. An oxide layer around particles of 10 to 20 microns average size may range in thickness up to about 2 microns.

The powder (e.g., 600 to 800 grams) may be directly hot pressed to the desired shape, or the powder charge may be first cold pressed to a cylindrical shape under 300 psi pressure and thereafter hot pressed to form the desired product. The hot pressing is carried out under vacuum (e.g., 5X10 Torr) at a pressure (18,000 psi) sufficient to provide a substantially fully dense sintered body. The hot pressing is preferably carried out at a temperature of about 1,050 C, the temperature being reached in 3 hours, held for 1 hour at temperature and then cooled to black heat in 2 hours. The die and punch assembly is shown schematically in FIG. 3 comprising a stress relieved die (16) of a molybdenum alloy containing by weight about 0.5% Ti, about 0.5% Zr, about 0.06% C and the balance essentially molybdenum. The die has a center opening or cavity 17 of about 2 inches in diameter running therethrough having fitted into it at opposite ends thereof punches l8 and 19 of the same alloy material. The assembly with the powder charge is placed in a vacuum chamber and heated to temperature by a conventional resistance heat susceptor.

In loading the die and punch assembly, a graphite foil disc 19A (0.005 inch thick) is placed on the inner end face of punch 19 within cavity 17 as shown; the cavity is lined with graphite foil to provide a sleeve thereof; and the powder material 22 then charged therein from the opposite end. Another graphite foil disc 18A is placed on top of the powder and punch 18 then inserted into the cavity in compression relationship to the powder charge. Pressure is applied to both punches in the direction of arrows 20 and 21 using a 30-ton capacity press. The graphite foil (discs and sleeve) help to eliminate contamination during the hot pressing cycle and also avoids fusion of the powder to the die and punches.

In applying the pressure after positioning of the die assembly with the vacuum heating chamber, a minimum pressure of lbs. (dial reading) is applied from room temperature to l,050 C, at which time the 18,000 psi load is applied and held for 1 hour. Thereafter, the power is shut off and the pressure released. As the pressed part reaches black heat, an Argon back fill is employed to obtain rapid cooling under inert conditions.

The density obtained is substantially equal to full density (for example, at least 99.9 percent of theoretical). The cylindrical product is cleaned by grinding or machining and the opposite faces are finished parallel for subsequent cutting into magnetic pole pieces. The final hardness on the Rockwell C" scale is generally at least about 50 R The preferred primary grain size of the final product lies within the ASTM 8 to l 1 range as measured in accordance with ASTM E112 at 100 times magnification.

In order to assure optimum magnetic properties, it is desirable that voids and inclusions not exceed a singular defect size greater than 10 microns. In addition, the distribution of defects should be such that the average distance between defects is at least 10 times greater than the maximum defect diameter.

A fully dense, hot consolidated product produced in accordance with the invention comprises approximately by weight 9.4 percent silicon, approximately 5.7 percent aluminum and the balance essentially iron. In addition, the alloy contains oxygen within the preferred range of 3,400 to 4,400 ppm, which provides a metallographic structure having the desired grain boundary oxide film exhibiting optimum wear resistance for magnetic pole pieces produced from the product. FIG. 4 illustrates such a microstructure. In determining the wear resistance of this powder metallurgy product, actual field testing was employed. Wear rates versus time data have disclosed that the powder metallurgy product exhibits at least a three to one improvement in pole piece life as compared to the same alloy in the conventionally cast form but without oxygen.

Summarizing the invention, a powder metallurgy method is provided of producing a hot consolidated magnetically soft ferrous alloy suitable for use in the manufacture of magnetic pole pieces. The ferrous alloy powder contains an effective amount of silicon and aluminum ranging from about 2 to 12 percent sili-con, from about 2 to 10 percent aluminum, about 2,000 to 8,000 ppm oxygen, and the balance essentially iron. The oxygen is added to the powder by pre-oxidation to provide a thin oxide film thereon and then hot consolidated at an elevated temperature sufficient to provide a substantially fully dense sintered body.

It is advantageous to employ milled and/or unmilled atomized powder (substantially flake-free) produced from vacuum cast alloy having a particle size range of about 325 mesh 5 microns and, more advantageously, from about 10 to 20 microns, the preferred alloy composition ranging by weight from about 9 to 9.8 percent silicon, about 5.4 to 6 percent aluminum, about 3,300 to 4,400 ppm oxygen, and the balance essentially iron. A particular composition is one containing approximately 9.4 percent silicon, approximately 5.7 percent aluminum, and the balance essentially iron. The sintered body is generally characterized metallographically by a grain size within the ASTM 8 to 11 range.

The preferred range of 3,300 to 4,400 ppm of oxygen is directly related to the improved wear resistance and magnetic performance of the alloy. However, as stated above, this range can be broadened for less stringent applications.

In vacuum hot pressing the alloy powder, the temperature may range from about 1,000 to l,l00 C with the preferred temperature being about l,050 C.

In hot pressing the thermally oxidized powder under vacuum, the pressure to achieve substantially full density may range from about 12,000 to 20,000 psi (e.g. 18,000 psi). As stated hereinbefore for a pressure of 18,000 psi, the preferred temperature of 1,050 C is reached in about 3 hours, the compressed powder being held at temperature for 1 hour and thereafter cooled to black heat in 2 hours. At the lower temperature range, a longer time of pressing is employed, while at the higher temperature range, a lower time interval is used.

In thermally oxidizing the powder in air, the temperature may range from about 450 to 650 F and, more preferably, from about 500 to 600 F.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

l.'A hot consolidated sintered magnetically soft ferrous alloy characterized by improved wear resistance when employed as a magnetic pole piece in rubbing contact with a moving magnetic playback tape of gamma iron oxide comprising,

a hot consolidated sintered powder metallurgy shape of said magnetically soft ferrous alloy consisting essentially of about 2 to 12 percent silicon, about 2 to percent aluminum, and the balance essentially iron,

said sintered alloy being characterized by a thin encapsulating film of thermally produced oxide sur rounding the grains thereof which confer improved wear resistance to said alloy,

the amount of oxygen ranging from about 2,000 to 8,000 ppm.

2. The sintered powder metallurgy alloy of claim 1, wherein the average grain size ranges from about 10 to 20 microns, wherein the composition consists essentially of about 9 to 9.8 percent silicon, about 5.4 to 6 percent aluminum and the balance essentially iron, wherein the oxygen content of said alloy rangest from about 3,300 to 4,400 ppm, and wherein the average grain size of said sintered alloy is within the range of about ASTM 8 to 11.

3. The sintered powder metallurgy alloy of claim 2, wherein the silicon content is approximately 9.4 percent and wherein the aluminum content is approximately 5.7 percent.

4. As an article of manufacture, a magnetic pole piece formed from a magnetically soft ferrous alloy characterized by improved wear resistance when employed in rubbing contact with a moving magnetic playback tape of gamma iron oxide,

said pole piece being formed of a sintered alloy composition consisting essentially of about 9 to 9.8 percent silicon, about 5.4 to 6 percent aluminum, and the balance essentially iron,

the grain size of said sintered alloy being within the range of about ASTM 8 to 1 l and being characterized by a thin encapsulated film of thermally produced oxide surrounding the grains thereof which confer improved wear resistance to said alloy, the amount of oxygen ranging from about 3,300 to 4,400 ppm.

5. The magnetic pole piece of claim 4, wherein the silicon content is approximately 9.4 percent and wherein the aluminum content is approximately 5.7

percent. 

2. The sintered powder metallurgy alloy of claim 1, wherein the average grain size ranges from about 10 to 20 microns, wherein the composition consists essentially of about 9 to 9.8 percent silicon, about 5.4 to 6 percent aluminum and the balance essentially iron, wherein the oxygen content of said alloy rangest from about 3,300 to 4,400 ppm, and wherein the average grain size of said sintered alloy is within the range of about ASTM 8 to
 11. 3. The sintered powder metallurgy alloy of claim 2, wherein the silicon content is approximately 9.4 percent and wherein the aluminum content is approximately 5.7 percent.
 4. As an article of manufacture, a magnetic pole piece formed from a magnetically soft ferrous alloy characterized by improved wear resistance when employed in rubbing contact with a moving magnetic playback tape of gamma iron oxide, said pole piece being formed of a sintered alloy composition consisting essentially of about 9 to 9.8 percent silicon, about 5.4 to 6 percent aluminum, and the balance essentially iron, the grain size of said sintered alloy being within the range of about ASTM 8 to 11 and being characterized by a thin encapsulated film of thermally produced oxide surrounding the grains thereof which confer improved wear resistance to said alloy, the amount of oxygen ranging from about 3,300 to 4,400 ppm.
 5. The magnetic pole piece of claim 4, wherein the silicon content is approximately 9.4 percent and wherein the aluminum content is approximately 5.7 percent. 